The present invention relates to a method for the preparation of a porous silica glass preform for optical fibers or, more particularly, to a novel method for the preparation of a porous silica glass preform as a precursor of optical fibers of the double-core type by the so-called VAD method, in which silica soot, i.e. fine particles of silicon dioxide formed by the flame hydrolysis of a vaporizable silicon compound, is deposited on the porous body under growing to form a preform for double-core dispersion-shift optical fibers having a center core, side core and cladding layer.
A double-core optical fiber above mentioned has a cross section having three concentrical portions including an innermost circular center portion, called the center core, having a refractive index n1, a ring-wise intermediate portion, called the side core, formed on and around the center core and having a refractive index n2 which is smaller than n1 and an outermost ring-wise portion, called the cladding layer, formed on and around the side core and having a refractive index n3 which is smaller than n2. The porous silica glass preform as a precursor of such a double-core optical fiber also has a concentrically structured cross section consisting of three portions including the center core, side core and cladding layer. The porous silica glass preform having such a ternary structure is prepared usually by the so-called VAD method using three burners held one above the other, of which the first, i.e. lowermost, second, i.e. middle, and third, i.e. uppermost, burners each serve to form the center core, side core and cladding layer, respectively, by the flame hydrolysis of a vaporizable silicon compound such as silicon tetrachloride with or without admixture of respective dopants for the control of the refractive indices in an oxyhydrogen flame formed on the nozzles of the respective burners.
Namely, the starting portion of the center core of the porous preform is first formed on a rotating refractory base by the flame hydrolysis using the first burner and, as the center core portion grows, the refractory base is gradually pulled up to hang the growing center core portion having a refractive index of n1 in a vertical position. As the center core portion grows, the silica soot produced in the oxyhydrogen flame on the second burner above the first burner is deposited on and around the growing center core portion to form the side core portion having a refractive index of n2 so that the porous body consisting of the center core portion and the side core portion grows in the vertical direction. Further, the silica soot produced in the oxyhydrogen flame on the third burner above the second burner is deposited on and around the side core portion under growing to form the cladding layer having a refractive index of n3. In the arrangement of the three burners one above the other, it is usual in the prior art that the extension of the nozzle axis of each of the three burners intersects with the axis of rotation of the porous silica glass body under growing.
While it is a requirement in a double-core silica glass optical fiber that the distribution of refractive index within the side core portion is uniform making clear demarcations with the center core portion and with the cladding layer or, namely, the distribution curve of refractive index in a radial direction taken in a cross section of the optical fiber has an ideally stepwise profile making definite stages for the respective portions, a problem in the doublecore optical fibers in the prior art prepared from the porous silica glass preform obtained in the above described method is that the refractive index of the side core portion is not uniform making no definite demarcations with the center core portion and with the cladding layer so that the profile of the distribution curve of refractive index in a cross section cannot be stepwise, in particular, in the side core portion. Needless to say, various proposals and attempts have been made heretofore in order to obtain an ideally stepwise profile of the distribution curve of the refractive indices in a radial direction within a cross section of the optical fiber but none of the proposals and attempts has been fully successful.